Telephone apparatuses, especially mobile phones, are often operated in noise polluted environments. Microphone(s) of the phone being designed to pick up the user's voice signal unavoidably pick up environmental noise, which leads to a degradation of communication comfort. Several methods are known to improve communication quality in such use cases. Normally, communication quality is improved by attempting to reduce the noise level without distorting the voice signal. There are methods that reduce the noise level of the microphone signal by means of assumptions about the nature of the noise, e.g. continuity in time. Such single-microphone methods as disclosed e.g. in German patent DE 199 48 308 C2 achieve a considerable level of noise reduction. Other methods as U.S. patent application 2011/0257967 utilize estimations of the signal-to-noise ratio and threshold levels of speech loss distortion. However, the voice quality of all single-microphone noise-reduction methods degrades if there is a high noise level, and a high noise suppression level is applied.
Other methods use an additional microphone for further improvement of the communication quality. Different geometries can be distinguished, which are addressed as methods with “symmetric microphones” or “asymmetric microphones”. Symmetric microphones usually have a spacing as small as 1-2 cm between the microphones, where both microphones pick up the voice signal in a rather similar manner and there is no principle distinction between the microphones. Such methods as disclosed, e.g., in German patent DE 10 2004 005 998 B3 require information about the expected sound source location, i.e. the position of the user's mouth relative to the microphones, since geometric assumptions are the basis of such methods.
Further developments are capable of in-system adaptation, wherein the algorithm applied is able to cope with different and a-priori unknown positions of the sound source. However, such adaption requires noise-free situations to “calibrate” the system as disclosed, e.g. in German patent application DE 10 2010 001 935 A1.
“Asymmetric microphones” typically have greater distances of around 10 cm, and they are positioned in a way that the level of voice pick-up is as distinct as possible, i.e. one microphone faces the user's mouth, the other one is placed as far away as possible from the user's mouth, e.g. at the top edge or back side of a telephone handset. The goal of the asymmetric geometry is a difference of preferably approximately 10 dB in the voice signal level between the microphones. The simplest method of this kind just subtracts the signal of the “noise microphone” (away from user's mouth) from the “voice microphone” (near user's mouth), taking into account the distance if the microphones. However since the noise is not exactly the same in both microphones and its impact direction is usually unknown, the effect of such a simple approach is poor.
More advanced methods use a counterbalanced correction signal generator to attenuate environmental noise cf. U.S. patent application 2007/0263847. However, a method like this is limited to asymmetric microphone placements and cannot be easily expanded to other use cases.
More advanced methods try to estimate the time difference between signal components in both microphone signals by detecting certain features in the microphone signals in order to achieve a better noise reduction results, cf. e.g., patent application WO 2003/043374 A1. However, feature detection can get very difficult under certain conditions, e.g. if there is a high reverberation level. Removing such reverberation is another aspect of 2-microphone methods as disclosed, e.g., in patent application WO2006/041735 A2, in which spectro-temporal signal processing is applied.
In U.S. patent application 2003/0179888 a method is described that utilizes a Voice Activity Detector for distinguishing Voice and Noise in combination with a microphone array. However, such an approach fails if an unwanted disturbance seen as noise has the same characteristic as voice, or even is an undesired voice signal.
U.S. patent application Ser. No. 13/618,234 discloses a two-microphone noise reduction method, primarily for asymmetric microphone geometries, and with suitable pre-processing also for symmetric microphones, however, it is then limited to a lateral focus (sometimes referred to as end-fire beam forming).
All of the methods or systems known in the art are either asymmetric in the definition of microphones, or—where symmetric microphones are used—they prefer an end-fire beam direction with the microphones behind each other.